1. Field of the Invention
The present invention relates to methods of measuring, adjusting and uniformalizing a sectional area ratio of a metal-covered electric wire, a method of cleaning an electric wire, a method of manufacturing a metal-covered electric wire, an apparatus for measuring a sectional area ratio of a metal-covered electric wire and an apparatus for electropolishing a metal-covered electric wire, and more particularly, it relates to a method of measuring, in a nondestructive manner, a sectional area ratio between a first material part and a second material part of a metal covered electric wire having a core part including the first material and a metal covering layer formed of the second material covering the core part, a method of adjusting the sectional area ratio by electropolishing and a method of uniformalizing the sectional area along the longitudinal direction, a method of cleaning a surface layer part of an electric wire having a metal surface by electropolishing, a method of manufacturing a metal-covered electric wire by electropolishing, an apparatus for measuring a sectional area ratio of a metal-covered electric wire in a nondestructive manner, and an electropolishing apparatus for dissolving a surface layer part of a metal-covered electric wire by electropolishing.
2. Description of the Background Art
Among metal-covered electric wires having core parts which are covered with metal covering layers, a metal superconducting wire such as a Cu-covered NbTi superconducting wire, for example, is formed by a stabilizing material of copper or a copper alloy and a superconducting material embedded therein for attaining circuit protection upon breakage of its superconducting state.
In that case, a wire having single core includes a core part consisting only of a superconducting material, and a covering layer formed of copper or copper alloy. Meanwhile, a wire having multiple cores includes a core part consisting of a superconducting material embedded in a stabilizing material, and a covering layer covering the core part.
A superconducting wire of such a structure is generally manufactured in the following manner, for example:
FIG. 13 is a flow chart showing steps of manufacturing an NbTi superconducting single-core wire. FIGS. 14 to 16 illustrate respective stages in the steps of manufacturing the NbTi superconducting single-core wire. Among these figures, FIGS. 14 and 16 are perspective views, and FIG. 15 is a sectional view.
Referring to FIGS. 13 and 14, an NbTi alloy rod 1 is prepared from a raw material and charged in a hollow cylindrical copper pipe 3 for serving as a stabilizing material for the single-core wire, and the copper pipe 3 is evacuated with a copper cover 5, and sealed by electron beam welding. A superconducting composite obtained in this manner is called a billet. This billet has a section which is substantially similar to that of the final single core wire.
Referring to FIG. 15, the wire diameter of this billet 7 is reduced to 50 to 30% through an extruder 9.
Referring to FIG. 16, an extruded body 11 as obtained is further reduced to a final wire diameter through a wire draw bench 13.
Then, the surface of the superconducting wire as obtained is cleaned. This step is generally carried out by acid cleaning for chemically dissolving the surface. In more concrete terms, the acid cleaning is performed by setting a vessel containing acid in a line having supply and take-up mechanisms and passing the wire through the line for continuously dipping the same in the acid, thereby dissolving its surface. The current amount of dissolution is adjusted by strength of the acid and a time for dipping the wire in the acid (length of the vessel and the wire speed through the line). The acid as employed is prepared from sulfuric acid for a Cu-covered superconducting wire, for example.
After the aforementioned cleaning, a single-core superconducting wire is obtained.
FIG. 17 is a flow chart showing steps of manufacturing an NbTi superconducting multicore wire. FIG. 18 is a perspective view showing a stage in the steps of manufacturing an NbTi superconducting multicore wire.
Referring to FIGS. 17 and 18, single-core wires 15 are formed into hexagonal sections through a die, cut and cleaned, and then a required number of such single-core wires 15 are simultaneously charged in a copper pipe 3, which in turn is covered evacuated with a copper cover 5 and sealed by electron beam welding, to manufacture a multicore billet.
Then, this billet is passed through an extruder similarly to the case of manufacturing a single-core wire, repeatedly wire-drawn and heat treated, and subjected to stranding and the like, to obtain a multicore superconducting wire.
In a Cu-covered superconducting wire as obtained, a sectional area ratio (Cu/SC sectional area ratio) of Cu or a Cu alloy to a superconducting material such as NbTi is generally called a copper ratio. This copper ratio, which is an important characteristic value showing stability of the superconducting wire, is finely specified in relation to application of the superconducting wire.
In general, this copper ratio, i.e., the Cu/SC sectional area ratio, is measured in the following manner, for example:
First, an end portion of a Cu composite superconducting wire is sampled and its weight is measured. Then, a stabilizing material part of Cu or a Cu alloy is removed from the Cu-covered superconducting wire, and the weight of the remaining superconducting material part is measured. The Cu/SC sectional area ratio is obtained from the total weight and the weight of the superconducting material part of the as-sampled Cu-covered superconducting wire by calculation.
On the other hand, this copper ratio is adjusted as follows: A superconducting composite called a billet can be regarded as having a section which is similar to that of the final target superconducting wire. Therefore, the thickness of a copper pipe and the amount of a superconducting material to be charged therein are adjusted to be equal to the final target copper ratio in manufacturing of the billet.
However, the aforementioned manufacturing of a superconducting wire has the following various problems:
When the copper ratio of a superconducting wire is measured by a conventional method, only a Cu/SC sectional area ratio in an end of a Cu-covered superconducting wire can be measured since the target portion is sampled and measured in a destructive manner.
When the Cu/SC sectional area is measured by such a conventional method in quality control of a Cu-covered superconducting wire as manufactured, for example, the overall CU-covered superconducting wire must be cut and removed if a value measured at its end portion is not in an allowable range. Thus, the yield is deteriorated in this case. In particular, the Cu/SC sectional area ratio is easily fluctuated at an end portion of a Cu-covered superconducting wire, and hence it has been regarded as problematic to estimate the Cu/SC sectional area of the overall Cu-covered superconducting wire from the value measured at the end portion. Therefore, dispersion of the copper ratio has been generally recognized through a destructive test along the overall length in an experiment, while such recognition cannot be applied to product inspection. Thus, the copper ratio in the middle of wire product cannot be measured.
In order to adjust the copper ratio of a superconducting wire by a conventional method, further, the copper ratio must be decided in manufacturing of a billet since only a sectional structure which is similar to that of the target wire is obtained by degressive working. Namely, it is necessary to manufacture billets having different copper ratios in order to manufacture superconducting wires having different copper ratios, and hence the manufacturing steps are complicated.
When a surface of an electric wire is cleaned or foreign matters are removed from the surface by a conventional method, on the other hand, the following problem arises: Namely, uniform dissolution cannot be attained in the conventional acid cleaning, since the dissolving power of the acid is reduced with cleaning. Further, an extremely long time is required for carrying out the dissolution which is required for removing foreign matters, due to insufficiency in absolute amount of surface dissolution in the same time.
When a superconducting wire is manufactured by a conventional method, in addition, ununiform deformation of the material is caused in a die applying a high pressure particularly when a billet is extruded, to cause dispersion in copper ratio of the as-manufactured superconducting wire.
It is assumed that such dispersion in copper ratio is caused in the following mechanism:
FIGS. 19 to 21 are sectional views showing states of a superconducting wire in extrusion.
When a billet comprising a core part 17, consisting of a superconducting material embedded in copper serving as stabilizing material which is covered with a copper cover portion 19 is extruded along arrow 23 through an extrusion die 21 as shown in FIG. 19, the copper cover portion 19 cannot satisfactorily flow with the core part 17 in the extrusion die 21 in an initial stage of extrusion, due to difference in strength and positional relation between the core part 17 and the copper cover portion 19.
Therefore, the copper cover portion 19 is rearwardly fed as if the same is scraped, as shown in FIG. 20.
Thereafter the rearwardly fed copper remains independently of the core part 17, to disadvantageously define the so-called bank 25, as shown in FIG. 21.
FIG. 22 is a longitudinal sectional view showing a state of the as-extruded superconducting wire.
Referring to FIG. 22, the copper cover portion 19 of the as-extruded superconducting wire includes a portion 29 which is thinned due to the rearward feeding of the copper, and the copper bank 25. Therefore, such a superconducting wire has dispersion in a sectional area ratio between a first material part and a second material part, and as a result, in copper ratio along its longitudinal direction.
The copper ratio is strictly defined in relation to application of the superconducting wire, as hereinabove described, so that a current flows into the copper serving as a stabilizing material when a superconducting state of the product is broken. If the copper ratio is dispersed along the longitudinal direction in the target wire diameter, therefore, the following problem arises:
When the copper ratio is low, the current cannot bypass toward the copper upon breakage of the superconducting state since the area of the copper is too small, and this may lead to a significant trouble such as burning of the wire. If the copper ratio is low, further, the area of the superconducting part is increased. Namely, a target draft cannot be sufficiently attained as to each superconducting filament, and hence a required critical current value may not be satisfied. When the copper ratio is high, on the other hand, the area of the important superconducting part is reduced and hence the superconducting part itself may not satisfy a required critical current value.